This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. At the present time, no vaccines to prevent infection with HCV are available and the epidemic continues unabated worldwide. The objectives of this study are to develop a candidate HCV vaccine consisting of HCV-like particles and to evaluate the immunogenicity of the vaccine in the chimpanzee model. Four HCV na[unreadable]ve chimpanzees were immunized with HCV-VLP vaccine formulated with adjuvants and immunostimulatory complexes. The vaccinated animals have been challenged with infectious HCV to determine the efficacy of the vaccine preventing infection. Recombinant hepatitis C virus (HCV)-like particles (HCV-LPs) containing HCV structural proteins (core, E1, and #2) produced in insect cells resemble the putative HCV virions and are capable of inducing strong and broad humoral and cellular immune responses in mice and baboons. Here, we present evidence on the immunogenicity and induction of protective immunity by HCV-LPs in chimpanzees. Chimpanzees (two in each group), were immunized with HCV-LPs or HCV-LPs plus AS01B adjuvant. After immunizations, all animals developed an HCV-specific immune response including IFN-[unreadable]+, IL-2+, CD4+, and CD8+ T cell and proliferative lymphocyte responses against core, E1, and #2. Upon challenge with an infectious HCV inoculum, one chimpanzee developed transient viremia with low HCV RNA titers (103 to 104 copies per ml) in the third and fourth weeks after the challenge. The three other chimpanzees became infected with higher levels of viremia (104 to 105) copies per ml), but their viral levels became unquantifiable (103 copies per ml) 10 weeks after the challenge. After the HCV challenge, all four chimpanzees demonstrated a significant increase in peripheral and intrahepatic T cell and proliferative responses against the HCV structural proteins. These T cell and proliferative responses coincided with the fall in HCV RNA levels. four na[unreadable]ve chimpanzees were infected with the same HCV inoculum, and three developed persistent infection with higher viremia in the range of 105 to 106 copies per ml. Our study suggests that HCV-LP immunization induces HCV-specific cellular immune responses that can control HCV challenge in the chimpanzee model.